The three-time Cup champion hit Kevin Ward Jr., who had gotten out of his car and approached Stewart’s as he drove around Canandaigua Motorsports Park under caution Saturday night, according to the Ontario (N.Y.) County Sheriff’s Dept. Ward was pronounced dead on arrival at Thompson Hospital about 45 minutes after being struck.

Regan Smith will replace Stewart for the Sprint Cup race today at Watkins Glen. Smith, who is racing full-time in NASCAR's Nationwide Series, has extensive Sprint Cup experience. The Cato, N.Y. native won the Southern 500 at Darlington Raceway in 2011 for Furniture Row Racing.

Officials of Stewart's Stewart-Haas Racing at first said Sunday morning that Stewart would race Sunday, but later announced that he would not. SHR competition director Greg Zipadelli said that Stewart made the decision Sunday morning.

“It’s just an unbelievable tragedy,” Zipadelli said. “Our hearts go out to Kevin and his family. Thoughts and prayers. This is a very tough, very emotional time for everybody.

“His family. Our family at Stewart-Haas. Tony Stewart. With that being said, we feel that as a group, Tony will not drive.”

NASCAR released a statement saying it supported Stewart's decision not to race.

“Our thoughts and prayers go out to the family, friends, and fellow competitors of Kevin Ward Jr.," the statement said. "We support Tony Stewart’s decision to miss today’s race and we will continue to respect the process and timeline of the local authorities and will continue to monitor this situation moving forward.”

Zipadelli announced the driver change at 10:15 a.m. ET, about three hours after the garage at Watkins Glen opened and the team prepared the car for Stewart to race. But Stewart apparently made the decision Sunday morning not to race. The team has not decided what it will do beyond this weekend.

“We gave Tony some time to sleep on it and met with him this morning and he feels strongly that this is the right thing to do,” Zipadelli said. “We at SHR support it and agree with it. It’s a difficult time for both parties.

“There’s not a lot you can do. The only thing we can do is do what we feel is right. This is what we feel is right and are supporting Tony in it.”

Smith, who drives for Dale Earnhardt Jr.'s JR Motorsports team in the Nationwide Series, will have to start at the rear of the field today. He finished 17th in the Nationwide race at the track Saturday. It’s not his first experience as a substitute as he replaced Earnhardt Jr. in 2012 when Earnhardt was sidelined by concussions.

“He’s a brother to me,” Zipadelli said of Stewart. “He’s way more than a boss. He’s going through a tough time. It’s emotional for him. … I support Tony Stewart. I think I’ve shown that over the last 18 years.”

Stewart has cooperated with the investigation, which is ongoing, according to Ontario County Sheriff Philip C. Povero. The sheriff, whose Sunday morning news conference was posted online by Rochester television station WROC, said his office was conducting an “on-track crash investigation” and the district attorney’s office has been notified. The results of the investigation, when completed by the sheriff’s department, will be shared with the district attorney as normal procedure with any death investigation, he said.

“There are no criminal charges pending at this time. … We are not making any inferences of any criminal conduct at all,” Povero said.

Povero stressed that Stewart had been cooperative in the investigation. He would not give specifics of Stewart's statement to police.

“He was visibly shaken by this incident and promised his continuing cooperation of this investigation,” the sheriff said.

Stewart, 19th in the Cup standings and winless this year, is set to start 13th.

“A tragic accident took place last night during a sprint car race in which Tony Stewart was participating,” SHR spokesman Mike Arning said in an statement early Sunday morning. “Tony was unhurt, but a fellow competitor lost his life. Our thoughts and prayers go out to his family and friends.”

Stewart was competing in the Empire Super Sprints event on the half-mile dirt track. The accident occurred on lap 14 of the 25-lap main event. The race was not resumed after the accident.

"Tony pinched him into the frontstretch wall, a racing thing," said eyewitness Tyler Graves, who had raced against the 20-year-old Ward in other sprint-car series. "The right rear tire went down, he spun on the exit of (Turn) 2. They threw the caution and everything was toned down. Kevin got out of his car. … He was throwing his arms up all over the place at Tony for most of the corner.

"I know Tony could see him. I know how you can see out of these cars. When Tony got close to him, he hit the throttle. When you hit a throttle on a sprint car, the car sets sideways. It set sideways, the right rear tire hit Kevin, Kevin was sucked underneath and was stuck under it for a second or two and then it threw him about 50 yards."

Stewart sponsor Bass Pro Shops also released a statement Sunday on the incident:

"All of us at Bass Pro Shops are all deeply saddened about last night's tragic accident at Canandaigua Motorsports Park. We send our thoughts and prayers to the family and friends of sprint car competitor Kevin Ward Jr and also to Tony Stewart and everyone at Stewart-Haas Racing."